I had a losing day after a long winning streak. I am sure it is because of over-confidence.

Don't read too much into daily results, those are mostly determined by variance.

I'm not exactly sure what "playing too confidently" means. One (and only) thing that comes to mind is that we overvalue our assumptions about how the game works and/or how our opponents are playing when we're actually just wrong or missing some major piece of the puzzle.

You're likely getting sloppy because you're winning. Eventually you'll kick yourself when the variance turns against you and you won less than you could've during your hot streak.

I don't think this can detected easily, or at all, through your HUD. I'd start by analyzing your big-medium pot hands at the end of the day. People tend to be bolder when they're more confident - so perhaps making too many river calls, or too many multi-barrels bluffs or river bluffs. You have to go through your own database to find these leaks to stop yourself.

Confidence is key in poker to execute your strategy repeatedly and without fear. Humility (and not lack of confidence) is what keeps a poker player hungry and working to improve.